For transmission of lift operation of a camshaft for opening/closing an intake and exhaust valve in a valve mechanism of four-stroke internal combustion engine, a tappet is used in a direct-hit type, and a rocker arm is used in a rocker arm type. The tappet or the rocker arm is provided between the camshaft and the intake and exhaust valve. When the valve is open, the tappet or the rocker arm is lifted while overcoming the reaction force of a valve spring. When the valve is closed, the tappet or the rocker arm is moved while pushed back by the valve spring, and the load from such spring force as well as the inertial force of the valve mechanism is constantly generated.
Recently, a rocker arm provided with a roller is widely employed for improved fuel efficiency. Such roller-type rocker arm includes four components in total, including a main body referred to as a body, an outer ring roller sliding with a camshaft, a shaft supporting the outer ring roller, and a small-diameter solid shaft referred to as a rolling element or a hollow roller referred to as an inner ring, which is between the shaft and the outer ring roller. The former using the rolling element is referred to as a rolling type, and the latter using the inner ring roller is referred to as a sliding type.
FIG. 1(A) shows a schematic perspective view of a sliding-type rocker arm, and FIG. 1(B) shows a schematic perspective view of a rolling-type rocker arm, with the body of the rocker arm omitted. A sliding-type rocker arm 10 includes a roller shaft 12, an inner ring roller 14 rotatably attached to the roller shaft 12, and an outer ring roller 16 rotatably attached to the outer surface of the inner ring roller 14. A rolling-type rocker arm 20 includes a roller shaft 22, a plurality of needle rollers 24 rotatably attached to the outer surface of the roller shaft 22, and a roller 26 a rotatably attached to the outer surface of the needle roller 24.
FIG. 2 shows an example diagram of a rolling-type rocker arm provided between a cam of a camshaft and a valve stem of an intake and exhaust valve. The rocker arm includes a body 30 holding rotatably a roller 26 as shown in FIG. 1(B), a first end 32 of the body 30 is supported by a pivot portion 34 and a second end 36 is abutted to a cap 38 of the valve stem of the intake and exhaust valve. The roller 26 is abutted to a cam 40, and the rotary motion of the cam 40 is transmitted to the body 30. Thus, according to the rotation of the cam 40, the second end 36 moves the intake and exhaust valve vertically. The sliding-type rocker arm is used in the same manner.
The rolling type has better friction performance compared to the sliding type because the rolling elements are rolling during operation. However, the rolling elements being slided are almost in line-contact with the shaft or the outer ring. Particularly, the rolling elements and the shaft have high contact pressure according to the Hertz's contact theory because the rolling elements have a small outer diameter which causes the contacts of both convex R to convex R.
In the sliding type, a lift load of a camshaft is supported by an inner circumference surface of an outer ring roller, an outer circumference surface of an inner ring roller, an inner circumference surface of the inner ring roller, and an outer circumference surface of a roller shaft. The inner ring roller and the roller shaft, which have the highest contact pressures, are used with lower contact pressure compared to rolling type because the inner ring roller has the wider inner diameter than the rolling element and thus the concave R to convex R contact is occurred for the roller shaft. Each sliding surface has a clearance and makes a relative motion while sliding. Then, the friction performance is de-graduated.
To improve fuel efficiency, there is a need to reduce the friction of the sliding portions. Also, to ensure the ability to smoothly transmit a lift operation for a long period, wear-resistance and scaffing-resistance are needed for the sliding portions. For such a conventional rocker arm, the techniques for suppressing wearing (see, JP2008-255883A) and for supplying lubricating oil efficiently (see, JP2007-023817A and JP2007-0263023A) have been disclosed. Further, the technique for avoiding damages or scaffings by providing lubricant film to a circumference surface of an inner ring roller (see, JP2000-034907A) has been disclosed.